1. Field
This patent specification relates to a constant voltage outputting method and apparatus capable of changing a rise time of an output voltage.
2. Discussion of the Background
In recent years, functions of a mobile device such as a mobile phone and a digital camera have become diverse, and performances and specifications of a power supply have also become diverse to keep up with the diverse functions of the mobile device. As a result, one mobile device needs to include a plurality of power supplies of different output voltages and different current capacities. Further, to prolong a running time of the mobile device, such control as to extend a battery life is performed by placing a circuit not used in a stand-by state or by turning the power supply off. Therefore, a plurality of power supply circuits are frequently activated and stopped within the mobile device.
Furthermore, a stabilized direct-current power supply device for performing a soft-start operation even when a reference voltage has risen is disclosed in Japanese Laid-Open Patent Publication No. 2003-216251. In this case, wherein a plurality of power supply circuits start operating at one time, if an output voltage rise time is substantially different among the plurality of power supply circuits, there arise such problems as a flow of an unintentionally large reactive current through the circuits and latch-up phenomenon occurring in the circuits. Therefore, the rise time of each of the plurality of power supply circuits should be determined so as to fall within a predetermined time period.
In FIG. 1, a typical background constant voltage circuit 100 includes an input terminal IN, an output terminal OUT, a reference voltage generation circuit 101, an error amplifier circuit AMPa, two resistors Ra and Rb, an output control transistor Ma, and an overcurrent protection circuit 102. The reference voltage generation circuit 101 generates and outputs a predetermined reference voltage Vref. The two resistors Ra and Rb detect an output voltage Vout. The constant voltage circuit 100 is connected to a load 110 via a bypass capacitor Ca.
A rise time of the output voltage Vout output from the constant voltage circuit 100 is determined mainly by combinations of current drive capacity of the output control transistor Ma, a value of a limited current of the overcurrent protection circuit 102, an amount of a phase compensation of the error amplifier circuit AMPa, a value of a load current flowing through the load 110, and capacitance of the bypass capacitor Ca.
Such factors as the value of the load current and the capacitance of the bypass capacitor Ca are different among circuits. Therefore, to set the rise time of the output voltage Vout output from the constant voltage circuit 100 within a predetermined time period, the value of the limited current of the overcurrent protection circuit 102 is adjusted by such techniques as a laser trimming in accordance with the value of the load current, the capacitance of the bypass capacitor, and so forth.
According to this background method of setting the output voltage rise time by using the laser trimming technique, however, circuit parameters of a constant voltage circuit are fixed, and thus versatility of the constant voltage circuit is diminished. As a result, in a circuit in which a different amount of the load current flows at every rise of a power supply circuit, even when the laser trimming is performed under a predetermined condition, if a condition under which the power supply circuit rises is changed, there arises a difference between an output voltage rise time of the power supply circuit and an output voltage rise time of another power supply circuit.